1. FIELD OF THE INVENTION
The present invention relates to a method and circuit for controlling an output of a dual-mode oscillator, the frequencies of which are produced by an SC-cut quartz crystal generating a B-mode signal and a C-mode signal.
2. DESCRIPTION OF THE PRIOR ART
Within the last decade, a new quartz crystal orientation known as the "SC-cut" has been developed. The SC-cut retains most of the mood qualities of the previous AT cut, such as low frequency-temperature coefficient, and it eliminates some of the bad qualities of the AT cut, such as temperature shock sensitivity. See "A Temperature Compensated SC-cut Quartz Crystal Oscillator" by Edward K. Miguel, 36th Annual Frequency Control Symposium (1982), pages 576 through 585.
Such crystals have found application in the development of dual-mode oscillators. For example, see "Dual-mode Operation of Temperature and Stress Compensated Crystals" by Kusters et al, pages 389 through 397. Whereas SC-cut crystals provide advantages over prior crystal orientations, they still require operation in a controlled temperature environment over a narrow temperature range in order to achieve good frequency stability.
Therefore, it would be considered desirable to develop a method and circuit for controlling the frequency of the output of a dual-mode crystal oscillator and, more particularly, for controlling the temperature of the dual-mode crystal oscillator so as to control the frequency of the output thereof.
The following patents are considered to be representative of the prior art relative to dual-mode oscillators: U.S. Pat. No. 4,079,280 [Kusters et al (Kusters '280)] and U.S. Pat. No. 4,215,308 [Kusters (Kusters '308)]. See, in particular, columns 2 and 3 of the Kusters '280 patent for a lengthy discussion of the prior art, said discussion incorporated by reference herein.
The Kusters '280 patent relates to a method and apparatus utilizing a quartz crystal resonator with an orientation substantially equal to 21.93.degree./33.93.degree. vibrating simultaneously in two thickness modes to accurately measure temperature and unknown frequencies or to provide a stable frequency signal source. By combining the quartz crystal resonator with either digital or analog compensation, the frequency-temperature deviation of one of the crystal mode frequencies is used as an internal thermometer and the second crystal mode frequency as a reference frequency signal. The frequency signal for the thermometer function is utilized by the compensation network to correct the measurements made with, or to stabilize, the frequency of the reference frequency signal. Additionally, the analog or digital compensation is accomplished by means of either a curve-fitting routine or an interpolation look-up table routine.
The Kusters '308 patent relates to a method and apparatus for obtaining an accurate measure of the frequency of an applied signal by generating a pair of independent frequency signals wherein one of the independent frequency signals has a frequency value which is indicative of the operating temperature of the counter. The counter has two modes of operation: (1) a calibration mode and (2) a measurement mode.
In the calibration mode, an accurate signal having a pre-selected frequency value is applied to the counter. The frequency of this signal is measured with reference to the other of the pair of independent frequency signals and this measured value is then algebraically combined with a prestored value that is equivalent to the pre-selected frequency of the applied signals. The combined value represents the necessary normalized correction factor for any measured frequency when the counter is at the corresponding temperature. The value of one of the independent frequency signals in this mode serves as a pointer to the memory location where this correction factor is to be stored. This procedure is then repeated several times as the temperature environment of the counter varies.
In the measurement mode, an unknown signal is applied to the counter, its frequency value is measured against the other of the independent frequency signals, the value of the one of the independent frequency signals is utilized as a pointer to the necessary correction factor at the then operating temperature of the counter, the measured frequency value is corrected algebraically with this correction factor, and the final result outputted for further use by the user.